1. Field of the Invention
This document relates to a method of fabricating organic light emitting diode display.
2. Related Art
In recent years, various kinds of flat panel display devices (“FPDs”) have been developed which are capable of reducing weight and volume which are disadvantageous in a cathode ray tube. Such FPDs include, for example, liquid crystal displays (“LCDs”), field emission displays (“FEDs”), plasma display panels (“PDPs”), electroluminescence displays (ELDs) and so on.
The PDPs are the most advantageous displays in implementing a slim, light-weight and large-sized screen since its structure and fabrication process are simple, but are disadvantageous in that they have low luminous efficiency and luminance, and large power consumption. The TFT LCDs (thin film transistor LCDs) have been most widely used but are disadvantageous in that they have a small viewing angle and a low response speed. The ELDs are largely classified into an inorganic light emitting diode display and an organic light emitting diode display according to materials used in an emission layer. Of the two, the organic light emitting diode display is a self-emitting element and is advantageous in that it has high response speed, luminous efficiency, and luminance, and a large viewing angle.
The organic light emitting diode display comprises an organic light emitting diode (“OLED”) as shown in FIG. 1.
The OLED, which is an organic electron element converting an electric energy into a light energy, has a structure where organic emission materials for emitting light are placed between an anode electrode ANODE and a cathode electrode CATHODE. Holes are injected from the anode electrode and electrons are injected from the cathode electrode. The holes and electrons are injected from the electrodes to an organic emission layer EML which emits light to thereby form excitons, and the OLED emits light due to energy generated when the excitons returns to a bottom level. In order to smoothly inject the holes and electrons into the emission layer EML from the electrodes, typically, a hole transport layer HTL and a hole injection layer HIL are placed between the emission layer EML and the anode electrode, and an electron transport layer ETL and an electron injection layer EIL are placed between the emission layer EML and the cathode electrode. For the smooth hole injection, the hole injection layer HIL and the hole transport layer HTL have an HOMO (highest occupied molecular orbital) level which corresponds to the middle level between the emission layer EML and the anode electrode. In addition, for the smooth electron injection, the electron transport layer ETL and the electron injection layer EIL have a LUMO (lowest unoccupied molecular orbital) level which corresponds to the middle level between the cathode electrode and the emission layer EML. Brightness and efficiency characteristics of the OLED element are determined by the amount of the holes and electrons injected from the anode electrode and cathode electrode. The amount of the holes injected from the anode electrode to the emission layer EML and the amount of the electrons injected from the cathode to the emission layer EML are varied depending on an energy level of the organic emission material.
Meanwhile, in the OLED display, for implementation of full colors, the emission layer EML is formed at a position where the OLED is disposed in each of red, green, and blue pixels. The emission layer EML is patterned for each pixel. As methods of forming the emission layer EML, there have been known a method of using a fine metal mask (FMM), an ink jet method, a laser induced thermal imaging (LITI), or the like.
In the FMM method, red, green, and blue emission materials are patterned for each pixel using a metal fine mask to form red, green, and blue pixels. This method has superiority in terms of element characteristics; however, it has a low yield due to the phenomenon of the mask blocking, and is hardly applied to a large-sized display device since a large-sized mask is difficult to develop.
The ink jet method is advantageous since the emission layer can be formed at selected regions and there is no damage to materials, thereby implementing large-sized screen and high definition, and enabling emission materials to have high luminous efficiency. However, in the ink jet method, there is need of accurate adjustment of an amount, a speed, a uniform jetting angle and so on of ink jetted from nozzles, and also, for implementing low cost and large-sized screen, there is need of development of ink jet heads for high speed jetting and increase of the number of heads. Further, quality and thickness of a thin film are required to be uniform so as to secure good emission inside pixel; however, there appears a so-called coffee stain effect where a periphery of the thin film becomes thicker in the course of drying ink drops, and thus the periphery is thickened.
The laser induced thermal imaging is a method in which a light source like a laser is irradiated to a transfer substrate formed of an organic emission material pattern, a light-to-heat conversion layer, and a support film, and the organic emission material pattern on the transfer film is transferred onto a substrate, thereby forming an emission layer. Describing this further in detail, in the laser induced thermal imaging, the transfer film provided with red, green, and blue organic emission material patterns is disposed on the substrate provided with black matrices, and thereafter the substrate and the transfer film are aligned and attached to each other. Next, the substrate to which the transfer film is attached is positioned on a stage of a laser irradiation device, and then the stage or a laser head moves from one end of the substrate to the other end thereof to perform a laser scanning. Thereby, a laser beam is sequentially irradiated to the red, green, and blue organic emission material patterns. Accordingly, the organic emission material patterns are sequentially transferred to the respective pixel regions on the substrate.
In the case where the organic emission layers are formed on the substrate by the use of the laser induced thermal imaging in this way, a series of processes are repeated to form the red, green, and blue organic emission layers, that is, the respective transfer films corresponding to the red, green, and blue are attached to the substrate, the laser is irradiated thereto in the scanning manner, and then the transfer films are detached. Thus, the repeated fabrication processes cause process time to be lengthened and the processes to be complicated. Further, there are problems in that bad patterns are sometimes generated due to micro bubbles in the course of attaching and detaching the respective transfer films of red, green, and blue, and the edges of the organic emission layers become rough by the repeated irradiation of the laser beam, and the attachment and detachment of the transfer films.